In U.S. Pat. No. 4,638,656, issued Aug. 12, 1986, in which the inventor of the present invention is a co-inventor, there is shown a pressure measuring instrument having means for monitoring the span calibration so that such calibration can be reset as necessary. This span calibration means comprises a reference pressure device for developing and applying to the instrument a reference pressure of precisely-controlled magnitude. In the preferred embodiment, the device includes a vertically-oriented cylindrical tube containing a fill-liquid which communicates with fill-liquid in the instrument. A solid metal ball in the tube can be raised to the top of the tube (as by an externally-developed magnetic field) and then released. The ball drops under the force of gravity through the fill-liquid to produce a pressure of essentially constant magnitude over much of the drop distance. This pressure is used as a reference pressure for setting the span calibration of the instrument, as by adjusting electronic circuitry forming part of the instrument, or located remotely from the instrument.
The "dropping ball" reference pressure device of U.S. Pat. No. 4,638,656 makes possible remote zero and span calibration of a pressure measuring instrument while the instrument is "in situ". The instrument is considered to be "in situ" when the instrument is mounted to the plant or pipeline under static process pressure condition and exposed to ambient temperatures and process temperatures, and other conditions of the plant or pipeline.
However, the accuracy of the calibration depends on both the reference pressure device and the pressure-measuring instrument being insensitive to temperature and static pressure. The "dropping ball" reference pressure device of U.S. Pat. No. 4,638,656 is sensitive to temperature and pressure because changes in temperature and pressure affect the specific weight of the fluid in the device, and change in specific weight affects the developed reference pressure.
More specifically, the "dropping ball" reference pressure device is an integral part of the instrument and is enclosed in silicone oil and is significantly influenced by changes in the buoyant force acting on the "dropping ball" caused by changes in process pressure and process temperature. The specific weight of the silicone oil changes by approximately 0.5% per 6.8 kPa (1000 psi) change in process pressure. The specific weight of the silicone oil also changes by approximately 5% per 55 degrees Celsius (100 degrees Fahrenheit) change in process temperature. To compensate for such changes, the process temperature and process pressure is measured and used to calculate the specific weight of the silicone oil. Then the calculated specific weight is used to correct the reference pressure. If not corrected, a change temperature as small as 5.5 degrees Celsius (10 degrees Fahrenheit) will produce a 0.5% error. Furthermore, because the "dropping ball" reference pressure device is an integral part of the instrument, the instrument contains a significantly higher volume of fill fluid than a comparable instrument without a "dropping ball" device. So changes in volume cause a correspondingly greater deflection of seal diaphragms. This results in a correspondingly greater sensitivity to temperature and pressure changes.
There is therefore, a need for a reference pressure device that is insensitive to temperature, static pressure and other sources of error.